The present invention relates to a sintering furnace for sintering an object to be sintered formed of ceramics, fine ceramic materials, etc. to produce a sintered object, a method of manufacturing a sintered object, and a sintered object.
Conventionally, an electric furnace or a gas furnace has been used as a sintering furnace for sintering an object to be sintered. However, since the temperature in a furnace must be raised gradually in order not to generate any temperature differences between the surfaces and the interiors of the object to be sintered in the case of such furnaces where the object to be sintered is heated from outside, there has been a problem that sintering time is liable to be longer.
Accordingly, in order to solve such a problem, Japanese Examined Patent Publication No. Sho 58-23349, Japanese Laid-Open Patent Publication No. Hei 3-257072 and Japanese Laid-Open Patent Publication No. Hei 6-87663 propose various sintering furnaces using microwaves. Microwaves are absorbed uniformly both onto the surfaces and into the interiors of the objects to be sintered. Therefore, there is only a little fear that any temperature differences occur during heating between the surfaces and interiors of the object to be sintered. Consequently, the rate of temperature rise can be increased to shorten the time necessary for the sintering to a large extent as well as uniform sintering can be accomplished. Sintering the objects to be sintered with microwaves is expected as such a technology that decreases energy necessary for the sintering as well as increases productivity of producing sintered objects, especially ceramics for industrial use.
The inventors of the present invention found that when sintering is conducted using microwaves, a pseudo-adiabatic space completely insulating an object to be sintered is created by surrounding the object to be sintered with a thermal insulating material which has an equivalent microwave absorption property to that of the object to be sintered. In this case, occurrence of thermal gradient in the object to be sintered due to radiation cooling can be restrained and more uniform sintering can be accomplished.
However, since energy of the microwaves is absorbed and consumed not only in the object to be sintered but also in the insulating material in the case of sintering the object to be sintered surrounded by the above insulating material, the amount of energy necessary for the sintering increases significantly.
In order to restrain the amount of energy consumed in the insulating material, it is necessary to make the insulating material thinner to decrease its weight and heat capacity. However, if the insulating material is made thinner, the amount of thermal energy lost by heat conduction out of the insulating material becomes larger to such a degree that cannot be neglected, compared to the amount of thermal energy given by the microwaves to the insulating material. Accordingly, there occurs a large temperature difference between the inside surface of the insulating material and the object to be sintered. Consequently, the above assumption of pseudo-adiabatic space will not be established resulting in occurrence of thermal gradient in the object to be sintered due to radiation cooling.
Therefore, a first object of the present invention is to provide a sintering furnace where occurrence of thermal gradient in an object to be sintered due to radiation cooling can be restrained while attempting to reduce energy necessary for sintering the object to be sintered, and a method of manufacturing an object to be sintered, and a sintered object.
For intending mass production of sintered objects, a tunnel type continuous sintering furnace is preferable where a plurality of processes can be carried out continuously. In the continuous sintering furnace, it is necessary to form an appropriate temperature distribution in the furnace by changing the temperature in the furnace in the direction of carrying the objects to be sintered. The reason is because each process (for example, drying, preliminary sintering, main sintering) of the sintering processes must be done in the particular temperature region within the furnace corresponding thereto.
However, in the case of the sintering with microwaves, formation of a proper temperature distribution in the furnace, which is a continuous cavity, is difficult, because electric power density of the microwaves is dispersed and uniformized through repetition of multiple reflection of microwaves within the furnace.
Therefore, a second object of the present invention is to provide a continuous sintering furnace where a temperature distribution corresponding to a plurality of processes can be easily formed in one furnace and an object to be sintered can be continuously sintered with microwaves in the furnace, and a method of manufacturing an object to be sintered, and a sintered object.